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Earthquakes | Volcanoes | Tornadoes | Hurricanes

Earthquakes

  • Earthquakes occur when vast plates, or rocks, within the Earth suddenly break or shift under stress, sending shock waves rippling.
  • Sudden movement along the fault causes the ground to shake.
  • Most earthquakes occur along fractures in the Earth's crust called faults.
  • Intraplate quakes occur far from plate edges and happen when stress builds up and the Earth's crust is stretched or squeezed together until it rips.
  • There are several different types of faults. Each can be a few inches or many hundreds of miles long. They can be horizontal, vertical, or at an angle.
  • Earthquake waves are measured on sensitive instruments called seismographs.
  • The Richter scale assigns quakes a number based on the power of its seismic waves.
  • Thousands of quakes occur every day around the globe, most of them too weak to be felt.
  • Every year about 10,000 people, on average, die as a result of earthquakes.

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Volcanoes

  • Volcanoes are vents in the Earth's surface from which molten rock, debris, and steam issue.
  • About 1,900 volcanoes are active today or known to have been active in historical times.
  • Almost 90 percent of volcanoes are in the Ring of Fire, a band of volcanoes circling the edges of the Pacific Ocean.
  • Most volcanoes occur at plate boundaries, areas where huge slabs of rock meet in the Earth's lithosphere, or outer shell.
  • Volcanoes can rise in subduction zones, areas where plates meet and one is pushed beneath another. Molten rock rises to the surface and forms a volcano.
  • Volcanoes can also arise in spreading centers, or rifts, where plates move away from each other, spreading or splitting the Earth's crust.
  • Intraplate volcanoes are caused by hot spots deep within the Earth. Magma rises and erupts as lava through cracks in the Earth's surface, forming volcanoes.
  • An eruption begins when magma, the molten rock from deep in the Earth's crust, rises toward the surface.
  • Volcanoes can erupt in a combination of ways: explosively with hard pyroclastic material; explosively with fluid lava (lava fountains); effusively with hard pyroclastic flows (clouds of ash and gases); and effusively with fluid lava.
  • Although some volcanoes are considered extinct, almost any volcano is capable of rumbling to life again.
  • Volcanoes provide valuable mineral deposits, fertile soils, and geothermal energy.

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Tornadoes

  • A tornado is a violently rotating column of air that descends from a thunderstorm to the ground.
  • Every U.S. state has experienced twisters, but Texas holds the record: an annual average of 120.
  • Tornadoes have been reported in Great Britain, India, Argentina, and other countries, but most tornadoes occur in the United States.
  • Tornadoes form when warm, humid air collides with cold, dry air.
  • The most violent tornadoes come from supercells, large thunderstorms that have winds already in rotation.
  • Most tornadoes in the United States occur in Tornado Alley, a swath that stretches from Texas to Nebraska and also includes Colorado, Iowa, Indiana, Illinois, Kansas, Missouri, and Arkansas.
  • Tornado season begins in early spring for the states along the Gulf of Mexico. The season follows the jet stream—as it swings farther north, so does tornado activity.
  • Twisters are usually accompanied or preceded by severe thunderstorms, high winds, or hail.
  • Once a tornado hits the ground, it may live for as little as a few seconds or as long as three hours.
  • The average twister is about 660 feet (200 meters) wide and moves about 30 miles (50 kilometers) per hour.
  • Meteorologists at the U.S. National Weather Service use Doppler radar, satellites, weather balloons, and computer modeling to watch the skies for severe storms and tornadic activity.

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Hurricanes

  • A hurricane is a rotating tropical storm with winds of at least 74 miles (119 kilometers) an hour.
  • These storms are called hurricanes when they develop over the Atlantic or eastern Pacific Oceans.
  • They are called cyclones when they form over the Bay of Bengal and the northern Indian Ocean.
  • They are called typhoons when they develop in the western Pacific.
  • Most Atlantic Ocean hurricanes form near the Cape Verde Islands off Africa's west coast.
  • Once a tropical storm's winds hit a constant speed of at least 74 miles (119 kilometers) an hour, it becomes a hurricane.
  • The eye is the low-pressure center of the hurricane. Air sinks inside the eye, clearing the skies and making it relatively calm.
  • A ring-shaped eye wall surrounds the eye and carries the storm's most violent winds and its most intense rains.
  • Hurricane season in the Atlantic, Caribbean, Gulf of Mexico, and central Pacific is from June 1 to November 30. In the eastern Pacific, it is from May 15 to November 30.
  • Hurricanes can cause floods, flash floods, tornadoes, and landslides.
  • Storm surge, an abnormal rise in sea level, is usually the most dangerous part of a hurricane. Surges can cause beach erosion, wash out roads, and decimate homes.
  • Forecasters at the U.S. National Hurricane Center in Florida use satellite imagery, airborne reconnaissance, and computer-model projections to track storms.

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